This application claims priority under 35 U.S.C. 119 of German application number P 44 09 073.0 filed Mar. 17, 1994.
The invention relates to a device for the handling of flowing liquids for analytical purposes.
One aspect is carrying out a chemical reaction at high temperature. The handling may, however, also consist in nebulization of the liquid to form an aerosol.
In particular, the invention relates to a device for the sample introduction in atomic spectroscopy, wherein a sample liquid under pressure is directed through a heated tube by means of a high pressure pump for forming an aerosol.
Another application of the invention is a device for carrying out a digestion process with a liquid at high temperature and under high pressure in a continuous flow.
The invention may also be used to carry out both a digestion process with a liquid and, subsequently, to form an aerosol for the sample introduction for atomic spectroscopy purposes, using the same apparatus.
A prior art method of nebulizing liquids consists in pressing the liquid by means of a high pressure pump through a heated tube. Such a device is called xe2x80x9cthermosprayxe2x80x9d. In such a device, the liquid is vaporized completely or partly. On the way of the liquid through the tube, at first, bubbles are progressively formed within the liquid. Then the flowing medium is predominantly vapor with less and less liquid droplets. In the end section of the tube, a vapor jet of high flow speed is formed. The tube is a narrow capillary.
In a prior art thermospray assembly (xe2x80x9cSpectrochimica Actaxe2x80x9d Vol. 43 (1988), 983-987) a quartz capillary encased by a steel tube is used. Pure vapor phase emerges from the exit of the quartz capillary. A similar assembly with a quartz capillary in a heated stainless steel tube for the sample introduction into an IPC is described in xe2x80x9cJournal of Analytical Atomic Spectrometryxe2x80x9d, Vol. 4 (1989), 213-217. Another assembly (xe2x80x9cSpectrochimica Actaxe2x80x9d Vol 41 (1986), 1287-1298) uses a directly heated metal capillary.
Published U.K. Patent Application No. 2,240,176 shows an apparatus for nebulizing liquids, for example from a liquid chromatograph, as aerosol into a mass spectrometer or some other gas detector. There, an aerosol is generated by pressing a liquid to be nebulized through an inner tube and pressing a well heat conducting gas such as hydrogen or helium through an outer tube concentric thereto. The gas is heated by a heater. Thereby, the liquid in the inner tube is vaporized, thermally nebulized droplets being formed similar to a conventional thermospray. This is a kind of combination of thermospray and pneumatic nebulization, which, according to the description, represents essentially a heated, pneumatic nebulization.
Furthermore, it is known to direct an aerosol emerging from a thermospray assembly by means of a carrier gas through a cooler, in order to condense the solvent. The condensed solvent is sucked off to a waste vessel at the bottom of a U-shaped cooler (company brochure xe2x80x9cSEPARATORxe2x80x9d of VESTEC Corporation, 9299 Kirby Drive, Houston, Tex. 77054).
Furthermore, it is known to de-solvatize aerosols, which are generated by a pneumatic nebulizer for the sample introduction for atomic spectroscopy, by vaporization, and to subsequently condense the vapor by a cooler. In this way, the liquid solvent is removed and a dry aerosol is obtained (xe2x80x9cSpectrochimica Actaxe2x80x9d Vol 23B (1968), 553-555). The same type of drying the aerosol is used in commercially available ultrasonic nebulizers for the IPC-spectrometry.
From German Patent No. 3,521,529 (=European Patent No. 0,208,901 B=U.S. Pat. No. 4,886,359) a device for nebulizing sample liquid for spectroscopic purposes is known, wherein a liquid to be nebulized is pumped at high pressure by a pump through a nozzle and is nebulized by the nozzle. In this apparatus, the pump is a high pressure pump designed as a separate assembly for generating a minimum pressure of 3 MPa (30 bar). Preferably, this is a continuously delivering multi-piston pump as used for high pressure liquid chromatography (HPCL). The nozzle connected to the pump through a conduit has a smallest cross sectional area for the flow of less than 1.3 10xe2x88x929 m2.
Published German Patent Application No. 3,026,155 shows a method of pneumatically nebulizing liquids by means of a pressurized gas stream concentric to a liquid carrying tube, wherein the nebulized liquid is subsequently vaporized by microwave radiation. According to Published German Patent Application No. 3,233,130, a liquid sample is vaporized from a carrier by supplying electrical energy. Solid samples are incinerated by applying infrared radiation. In this way a dry aerosol is generated and is supplied to a spectrometer.
Furthermore, digestion processes under high pressure and at high temperature for analytical purposes are known. In such processes, the liquids to be digested are filled into thick-walled containers of stainless steel which contain an inert inner container of PTFE (digestion bomb). These containers are closed by threaded caps. Then the containers are heated by a heater. This is a batch process. This process is time consuming since the containers cannot be opened before they have been cooled down.
It is the object of the invention, to provide an advantageous device for handling flowing liquids, which permits the liquid to be kept at high temperature.
More specifically, it is an object of the invention to improve the sample introduction in spectroscopy.
A further, still more specific object of the invention is to facilitate and speed up the digestion process of liquids for analytical purposes.
According to the invention, this object is achieved by a device for handling flowing liquids, comprising a high pressure pump for feeding the liquid, a cavity connected with the outlet of the high pressure pump, means for heating the cavity and a restrictor connected in series with the cavity at the outlet side thereof, the restrictor being dimensioned relative to the delivery of the high pressure pump such that it ensures a substantially elevated pressure in the cavity as compared to atmospheric pressure.
Such a device permits heating of a liquid in the cavity in through-flow to high temperatures without development of vapor. Development of vapor is prevented by the high pressure generated by the high pressure pump. In the assembly of the invention, a quasi-closed system is provided, in which the liquid is under a pressure which lies above the saturated vapor pressure of the liquid. Under this pressure, the liquid can be heated to high temperatures. Preferably the cavity has such a low flow resistance as compared with the flow resistance of the restrictor, that a substantially constant pressure prevails in the cavity over the length thereof.
If the liquid emerges through a restrictor designed as a nozzle and is expanded thereby, part of the super-heated liquid is vaporized. Then two influences cooperate in finely nebulizing the liquid: On one hand, the liquid is nebulized, when leaving the nozzle, by xe2x80x9chigh pressurexe2x80x9d nebulization of the type described in German Patent No. 3,521,529. On the other hand, however, there is also spontaneous nebulization by vaporization of part of the liquid. Thereby, a very finely nebulized aerosol can be generated. The yield of the sample for introduction into a spectrometer is improved.
With another use of the assembly of the invention, the high temperature which can be achieved under high pressure is used to initiate chemical reactions, for example to carry out a digesting process. This is done continuously in through-flow. As no vapor develops, no crystallized depositions occur even when using highly concentrated salt solutions.
Modifications of the invention are subject matter of the dependent claims.
Embodiments of the invention are described in greater detail hereinbelow with reference to the accompanying drawings.